Photographic printing apparatus



July 11, 1944. B; BURNHAM ET A1. 2,353,218

PHOTOGRAPHIC PRINTING APPARATUS Filed March 14, 1941 Fl E I 115 x H NEGATlVE I I AUXILIARY BEAM ' SPLITTER 3 J EXPOSURE LEVEL 1 ADJUSTMENT PHOTOSENSITIVE l MATERIAL A 1 v 12 v FIG 2.. FIG. 5

18 PRIOR ART FIG. 4. FIG. 5.

BRADSHAW BURNHAM EBEN HDAG INVENTORS mxmzz.

A TTORNE YS Patented Jo 11,-1944 PHOTOGRAPH IC PRINTING APPARATUS Bradshaw Burnham and Eben Hoag,.Rochestcr, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application March 14, 1941, Serial No. 383,376

6 Claims.

trolling characteristic of a medium being processed.

More specifically, the invention relates to apparatus for controllingvautomatically the exposure given a sensitive photographic layer.

In exposing photographic layers to light of different average intensities it is generally desirable to give each layer a constant exposure so that, upon being developed, a record of substantially constant density will be produced. However, due to failure of the reciprocity law, it is known to be desirable to increase long exposures so that the quantum of exposure light progressively increases as the average intensity of theexposure decreases.

In photographic printing, especially where the exposure is given by the turning on and off of an incandescent lamp, it has been found that when the lamp switch is turned off at the proper time for correct exposure the actual exposure will be more than is necessary and the shorter the exposure interval the greater will be the over exposure so that when very thin negatives are printed the over exposure will render the prints unsatisfactory.

It is an object of the present invention to provide a printing apparatus which will automatically increase the quantum of printing light as the average transmission of the record being printed decreases. Another object of the invention is to provide an automatic printer in which short exposures given thin negatives are appreciably shortened so as to take into account and to compensate for the exposing light radiated by the printing lamp after it has been turned off.-

In carrying the invention into effect the exposure timing mechanism is made responsive to the average transmission of the record to be printed and is provided with a pre-trip device or afterglow anticipator which is effective only when the exposures are very short. The timing mechanism is also provided with an exposure lengthening arrangement which becomes effective only when relatively long exposures are involved.

In the specific embodiment of the invention about to be described the variation of the charge on a condenser, in accordance with the printing light employed .for timing the exposure, is nodifled or caused to function in a manner to effect progressively shorter and longer exposures, respectively.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, will best be understood from the following description when read in connection with the accompanying drawing in which:

Fig. 1 shows diagrammatically a printer having an automatic exposure control arrangement incorporating the preferred embodiment of the invention;

Fig. 2 is a fragmentary showing of a prior art circuit;

Figs. 3 and 4 are fragmentary diagrams of two forms of afterglow anticipating circuits; and,

Fig. 5 is a fragmentary diagram of a modification of the arrangement shown in Fig. 1.

The basic exposure control circuit arrangement in which'the invention is incorporated for the purpose of illustration is one adapted for use with a, direct current supply and its design follows closely the systems described and claimed in Merriman and Burnham application Serial No. 309,222, filed December 14, 1939, Patent No. 2,258,994.

Referring to the drawing there is shown dia-, grammatically in Fig. 1 a printer comprising a lamp l0 positioned to illuminate a record II to be printed, which record is imaged on photographic material |2 by a suitable lens Ill. The starting and stopping of an exposure interval may be obtained in any suitable manner and is here shown as being controlled by a switch It which is biased to circuit closing position by a spring |5 and is adapted to be held in open circuit position by arelay coil l6 whenever the coil I6 is energized. This coil I6 is included in the plate circuit of a suitable electronic tube l1 shown as having a cathode l8, an anode l9 and a control electrode or grid 2|]. Thus, whenever plate current is flowing through the tube II, the

relay I6 is energized and the lamp lilis dark. For the reasons appearing hereinafter, it is preferred to include an auxiliary lamp 2| in series with the lamp Ill and so selected that the current therethrough will cause the lamp 2| to emit light but is not sufficient to cause the lamp l0 to glow. The switch l4, when closed, shunts the lamp 2| and full voltage is applied to lamp I0 which brings it to incandescence.

With the power leads 22 and 23connected to any suitable source of direct current ajPIihtin control switch 24 which maybe controlltifili! the platen (not shown) will be in engagement with a contact 25 which may connect a resistance 25 between the cathode l8 and the anode l9 to insure the existence of plate current for energizing the relay IE to hold the switch l4 open. With the switch 24 in engagement with the contact 25 a potentiometer 21 having an adjustable contact 28 supplies the grid 29 of the tube I! with a potential corresponding to the exposure level desired. With this potential on the grid 29 the grid current will charge a condenser 29 which has included in its discharge path a lightsensitive cell 30 arranged to receive a predetermined portion of the light transmitted by the record I I by means of a transparent mirror 3|.

This basic control circuit is shown in simplified form in Fig. 2 and is well known. It functions to control the exposure interval in the following manner. A predetermined charge is maintained on the condenser 29 so that when the switch 24 i moved to the position shown in Fig.1 to start a printing interval, this movement of the switch 24 removes the charging potential from the condenser 29 which causes-the grid 20 to become sufliciently negative to stop the flow of plate current through the tube I! which deenergizes the relay coil l6 and permits the spring l to close the switch l4, thereby energizing the lamp Hi. When the lamp I0 is energized to start an exposure, a portion of the light is directed onto the cell 30 by the mirror 3| to cause it to assume a resistance corresponding to the transmission of the record II. The condenser 29 commences to discharge through the cell 39 and at a rate dependent upon the resistance of the cell 39 until the charge on the condenser 29 has been reduced to a value such that the potential of the grid 29 no longer prevents the flow 01 plate current through the tube ll. When this late current again starts to flow, the relay I8 is actuated to darken the lamp H) to discontinue the exposure.

When the lamp I9 is deenergized, as above described, it continues to emit light for an appreciable time while it is cooling off and when very short exposures are involved, i. e., when printing very thin negatives, this afterglow of the lamp I9 causes the print to be overexposed. In accordance with one feature of the invention, the

control circuit is designed to anticipate this afterglow and to bring about a progressive shortening of short exposures.

As shown in Fig. 3, this desired shortenin of short exposures may be brought about by including in series with the control condenser 29 a resistance 32 so that it is the voltage across this series combination which is impressed between the cathode l9 and the grid 20 of the tube H. This arrangement functions to shorten short exposures for the reason that the thinner the rec- 0rd II, the smaller is the resistance of the cell 39 and, therefore, the current discharging the condenser 29 is larger. With a relativel large discharging current flowing through the series resistance 32, its IR drop will be appreciable and,

since the tube I1 is sensitive to the voltage acrossthe condenser 29 plus this IR drop, it follows that the switch l4 will be opened to stop an exposure before the condenser 29 has reached the condition predetermined to give, the desired exposure level for records of average transmission. When records of average transmission are printed, the resistance of cell 30 is relatively high and the discharge current for the condenser 29 is correspondingly low so that the 1R drop across the resistance 32 is negligible.

The control circuit just described is prone to be unstable, but this dii'ilculty may be satisfactorily overcome by shunting the resistance 32 with an auxiliary condenser 33 as shown in Fig. 4. With this arrangement large discharging currents accompanying short exposures discharge the condensers 29 and 33 so rapidly that the resistance 32 doesnot appreciably affect the condenser series connection and an exposure results approaching that which would occur if a single condenser were used having a capacity equal to the series capacity of the two series connected condensers 29 and 33. For smaller values of current involved in longer exposures the resistance 32 renders the auxiliary condenser 33 decreasingly effective and exposures result approaching that which would occur if the main exposure control condenser 29 were used alone.

As is well known, the exposure level, which produces satisfactory prints from average negatives, will result launder-exposed prints when very dense negatives are printed. In order to progressively increase the duration of long exposures, the control condenser 29 is,according to the invention, and as shown in Fig. 5, shunted by a condenser 34 and a resistance 35 in series. The capacity of the condenser 34 and the magnitude of the resistance 35 are such that, for short exposures, the condenser 34 does not appreciably affect the exposure interval, while, for longer exposures, the condenser 34 becomes increasingly effective, thereby securing a greater current-time product which, for very small values of current through the cell 33, approaches that of the two condensers 29 and 34 in parallel. During the periods between exposures the resistance 35 may be shorted by a switch 36 to eliminate any delay in the condenser 34 being fully charged. This switch 39 may be operatively cou- 31 as shown in Fig. 1 arranged to receive light from the auxiliary lamp 2|. As was pointed out 7 above, this lamp 2| is energized onl during the time a printing exposure is not taking Place.

The preferred embodiment of the invention, as shown in both Figs. 1 and 5, incorporates in its control circuit both the arrangement for shortening short exposures and the arrangement for lengthening relatively long exposures, 50 that at each end of the exposure time scale the control circuit provides a printing interval which is a variable function of the intensity of the exposure.

While the invention has been described in connection with a specific embodiment, it will be obvious to those skilled in the art that the invention may be practiced in various otherways without departing from the spirit of the invention, the scope of which is pointed out in the ap-' voltage across the condenser, and means connected in shunt relation with the condenser for decreasing the rate of discharge of the condenser faster than the resistance of the discharge path increases.

condenser whose charge is varied, a circuit for impressing upon the voltage responsive means the voltage across the condenser and its series reslstance, whereby when the condenser charge is varied at a rapid rate the exposure controlling voltage is noticeably augmented by the IR drop across the resistance, said IR drop becoming decreasingly effective as the rate the condenser charge is varied decreases.

4. In photographic printing apparatus in which the exposure is automatically controlled, a condenser, means for varying the charge on the condenser as a function of the average transmission of the record being printed, whereby the voltage across the condenser is varied, voltage responsive means for terminating the printing 7,

5. A system for providing time intervals corresponding to the magnitude of a condition, comprising a control means responsive to a predetermined voltage across its terminals for terminating a time interval, means for impressing a standard voltage across said terminals, a circuit including a condenser and an associated impedance connected across said terminals, means .for initiating a. time interval, means operative upon the initiation of the time interval for causing to flow through said circuit a. current proportional to the magnitude of the condition, whereby the voltage across said terminals is varied, said associated impedance being so proportioned and connected that when the voltage across said terminals reaches said predetermined value the voltage across said condenser differs therefrom as a function of the current flowing through the circuit.

6. In photographic printing apparatus in which the variation of an electric potential through a predetermined range and under the control of a light sensitive cell determines the exposure period, a main condenser and an auxiliary condenser in series with a light sensitive cell, a control circuit responsive to a. predetermined potential across the two condensers for timing the exposure, and a resistance connected in shunt relation to the auxiliary condenser, whereby as the resistance of said cellincreases from a minimum the effective combined capacity of the two condensers, increases from substantially their series capacity to a value approaching the capacity of the main condenser alone.

BRADSHAW BURNHAM. EBEN HOAG. 

